global start

section .text
bits 32
start:
	mov esp, stack_top

	call check_multiboot
	call check_cpuid
	call check_long_mode

	; print 'OK'
	mov dword [0xb8000], 0x2f4b2f4f
	hlt

check_multiboot:
	cmp eax, 0x36d76289
	jne .no_multiboot
	ret
.no_multiboot
	mov al, "0"
	jmp error

check_cpuid:
	; Check if CPUID is supported by attempting to flip the ID bit (bit 21)
    ; in the FLAGS register. If we can flip it, CPUID is available.

    ; Copy EFLAGS in to EAX via stack
	pushfd
	pop eax

	; Save the current flags
	mov ecx, eax

	; Flip the ID bit
	xor eax, 1 << 21

	; push eax to eflags
	push eax
	popfd

	; Copy EAX to FLAGS via the Stack
	pushfd
	pop eax

	; Restore FLAGS from the old version stored in ECX (i.e. flipping the
    ; ID bit back if it was ever flipped).
    push ecx
    popfd

	; Check if the ID was changed
	cmp eax, ecx
	je .no_cpuid
	ret
.no_cpuid:
	mov al, "1"
	jmp error

check_long_mode:
    ; test if extended processor info in available
    mov eax, 0x80000000    ; implicit argument for cpuid
    cpuid                  ; get highest supported argument
    cmp eax, 0x80000001    ; it needs to be at least 0x80000001
    jb .no_long_mode       ; if it's less, the CPU is too old for long mode
	
	; extended info about long mode
    mov eax, 0x80000001    ; argument for the cpuid function
    cpuid                  ; Cpu id
	test edx, 1 << 29  	   ; check long mode availablity
	jz .no_long_mode
	ret

.no_long_mode
	mov al, "2"
	jmp error

; Prints 'ERR: ' and the given error code to the screen and halts
; parameter: error code letter (ascii) in al
error:
	mov dword [0xb8000], 0x4f524f45
	mov dword [0xb8004], 0x4f3a4f52
	mov dword [0xb8008], 0x4f504f20
	mov byte  [0xb800a], al
	hlt

section .bss
stack_bottom:
	resb 64
stack_top: